1. Field of the Invention
The invention relates generally to semiconductor integrated circuit devices operating at a high frequency, and more particularly, to a semiconductor integrated circuit device which can decrease reflection of transmitted signals in wiring. The invention has particular applicability to microprocessors operating at a high frequency exceeding 100 MHz.
2. Description of the Background Art
With the recent increase in degree of integration in a semiconductor integrated circuit device, requirements for operation at high speed are more and more increasing. For example, a microprocessor currently well known operates with a basic operation clock signal of 40 MHz, and it is expected that operation processing speed will be more and more increased in the future.
With the increase in operating speed of a semiconductor integrated circuit device, the presence of reflected signals generated in wiring cannot be ignored. For example, when a semiconductor integrated circuit device is operated with a basic operation clock signal exceeding 100 MHz, reflection of transmitted signals is generated in various parts in wiring. Reflection of transmitted signals is generated in areas where impedance of wiring is changed discontinuously, such as in connection portions of wiring using contact holes, portions where wiring width is decreased and the like.
Reflection of transmitted signals generated in wiring not only decreases energy of transmitted signals, but also causes malfunction in an electronic circuit of destination, because reflected signals are transmitted there. Although the present invention is applied to a semiconductor integrated circuit device operating at a high frequency exceeding 100 MHz, the following description will be made on a microprocessor as an example.
FIG. 7 is a general block diagram of a microprocessor. The block diagram shows schematically a layout of a circuit configuration formed on a semiconductor substrate 50. Referring to FIG. 7, the microprocessor is provided with a bus control circuit 61 for controlling an internal data bus and an internal address bus, an instruction decoder 62 for decoding an instruction code IC, applied from bus control circuit 61, a control circuit 63 for generating various control signals Sc responsive to a decoded instruction code and an operation executing circuit 64 for carrying out operation responsive to control signal Sc. Data and address signals used in operation are applied to operation executing circuit 64 through bus control circuit 61. Data and address signals are transmitted through a wiring 65 formed on semiconductor substrate 50. On the other hand, control signal Sc is transmitted through a wiring 66 formed on substrate 50. It is pointed out that these wirings 65 and 66 have generally long wiring length.
FIG. 8 is a circuit block diagram for explaining transmission of signals in a conventional semiconductor integrated circuit device. Referring to FIG. 8, the semiconductor integrated circuit device includes two CMOS circuits 1 and 4 formed on a semiconductor substrate 51. Each of CMOS circuits 1 and 4 is configured by CMOS transistors (not shown). An inverter 2 for transmission is provided at an output of CMOS circuit 1. An inverter 3 for reception is provided at an input of CMOS circuit 4. A wiring 12 is provided between inverters 2 and 3. In the following description, it is assumed that a transmitted signal Sp output from CMOS circuit 1 is transmitted to CMOS circuit 4 through wiring 12. It is pointed out that wiring 12 shown in FIG. 8 corresponds to one of wirings 65 and 66 in a microprocessor, for example, shown in FIG. 7.
As mentioned before, wiring formed on semiconductor substrate 51 connects CMOS circuits 1 and 4 via a long wiring path on substrate 51. The long wiring path includes connection portions using contact holes and/or portions where wiring width is decreased. In such portions, impedance of wiring 12 is partially changed in general. In other words, impedance of wiring 12 is changed discontinuously. A resistance 14 shown in FIG. 8 equivalently shows resistance components generated in, for example, connection portions of wiring using contact holes. Impedance of wiring 12 is, as described before, changed discontinuously due to change in form of wiring and, therefore, such portions of discontinuous impedance are caused in a plurality of portions in a long wiring path. However, for simplification of description, only one portion is shown in FIGS. 8.
As shown in FIG. 8, although transmitted signal Sp is transmitted toward CMOS circuit 4 through wiring 12, a part of energy of transmitted signal Sp is reflected due to the presence of resistance component 14. Therefore, a reflected signal Sr reflected by resistance component 14 is superimposed on wiring 12. Other components (main components) St of transmitted signal Sp are transmitted toward CMOS circuit 4 through resistance component 14.
The reason why reflection of transmitted signal Sp is generated is described in the following. Since transmitted signal Sp is a digital clock signal of a high frequency exceeding 100 MHz, various signal components of a high frequency are included therein. It is understood that wiring 12 has characteristic impedance in signal transmission at a high frequency. Therefore, the presence of resistance component 14 serves as a discontinuous point of characteristic impedance in wiring 12. In other words, the presence of resistance component 14 causes mismatching in wiring. Reflection of transmitted signal Sp is generated in such a mismatching portion in impedance.
Reflected signal Sr causes the following problems. First, since reflected signal Sr having a high frequency is also transmitted to CMOS circuit 4 of destination, it causes malfunction in CMOS circuit 4. In other words, reflected signal Sr transmitted to CMOS circuit 4 is superimposed on original signal St and serves as a noise with respect to CMOS circuit 4. As a result, malfunction is caused in CMOS circuit 4.
In addition, since transmitted signal Sp is a clock signal, i.e., a pulse signal, having a high frequency, it includes various high frequency components. More specifically, signal Sp includes wideband signal components. As described before, since signal components of a high frequency are partially lost from transmitted signal Sp by reflection, this causes a waveform of transmitted signal Sp to change. In other words, steep rise or fall of transmitted signal St is lost. It is pointed out that this also causes malfunction in CMOS circuit 4.
Generally speaking, since clock signals or pulse signals processed in a digital circuit are wideband signals, points of discontinuous (or mismatching) impedance are ubiquitous in a circuit. This means that reflection may be generated everywhere in a circuit. As a result, malfunction is easily caused by reflection in a high speed digital circuit.